Method, apparatus and system for reducing gain ripple in a raman-amplified WDM system

Abstract

A method, apparatus and system for reducing gain ripple in a Raman-amplified WDM system includes determining a gain profile of an optical signal transported via a first Raman amplification span in a Raman-amplified WDM system, comparing the determined gain profile to a gain profile of an optical signal transported via a second Raman amplification span, determining, from the comparison, an amount of wavelength adjustment required for a plurality of Raman pumps comprising at least one Raman pump block of the second Raman amplification span to alter the gain profile of an optical signal transported via the second Raman amplification span, such that a cumulative gain profile of an optical signal transported via the first and the second Raman amplification spans approaches a desired gain profile, and generating a control signal suitable for adjusting the wavelengths of the plurality of Raman pumps of the at least one Raman pump block an amount consistent with the determined amount.

1. A method, comprising:
 determining a gain ripple profile of an optical signal propagated via a first Raman amplification span in a Raman-amplified WDM system;  
 comparing said determined gain ripple profile to a gain profile of a second Raman amplification span;  
 determining, from said comparison, an amount of wavelength adjustment required for a plurality of Raman pumps comprising at least one Raman pump block of said second Raman amplification span to alter the gain profile of said second Raman amplification span, such that a cumulative gain profile of an optical signal transported via said first and said second Raman amplification spans approaches a desired gain profile, wherein said desired gain profile is a gain profile tending to reduce the gain ripple of at least said optical signal in said first Raman amplification span; and  
 generating a control signal suitable for adjusting the wavelengths of said plurality of Raman pumps of said at least one Raman pump block an amount consistent with said determined amount.  
 
   
   
     2. The method of  claim 1 , further comprising:
 adjusting, substantially simultaneously, the wavelengths of said plurality of Raman pumps of said at least one Raman pump block an amount consistent with said determined amount.  
 
   
   
     3. The method of  claim 1 , wherein said desired gain profile is a relatively flat gain profile. 
   
   
     4. The method of  claim 1 , wherein said desired gain profile is a predetermined gain profile. 
   
   
     5. The method of  claim 1 , further comprising:
 determining a cumulative gain profile of an optical signal transported via said first and said second Raman amplification spans.  
 
   
   
     6. The method of  claim 5 , wherein said adjusting is iteratively performed until said cumulative gain profile further approaches said desired gain profile. 
   
   
     7. The method of  claim 1 , wherein the wavelengths of said plurality of Raman pumps are adjusted by adjusting the temperature of said at least one Raman pump block. 
   
   
     8. The method of  claim 1 , wherein each Raman pump of said plurality of Raman pumps is a tunable Raman pump. 
   
   
     9. The method of  claim 8 , wherein the wavelengths of said plurality of Raman pumps are adjusted by tuning the wavelength of said each Raman pump. 
   
   
     10. The method of  claim 1 , wherein said first Raman amplification span comprises at least one Raman amplification fiber wherein Raman amplification is provided to said optical signal. 
   
   
     11. The method of  claim 10 , wherein said Raman amplification fiber comprises a standard transmission fiber. 
   
   
     12. The method of  claim 1 , wherein said first Raman amplification span comprises at least one Raman amplification fiber and at least one dispersion compensating fiber, wherein Raman amplification is provided to said optical signal. 
   
   
     13. The method of  claim 12 , wherein said gain profile of said optical signal transported via said first Raman amplification span comprises a cumulative gain profile of said at least one Raman amplification fiber and said at least one dispersion compensating fiber. 
   
   
     14. The method of  claim 1 , wherein said determining an amount of wavelength adjustment, comprises:
 matching the gain profile of said optical signal transported via said first Raman amplification span to a sine function, said sine function having a wavelength period of Δλ period ;  
 determining the location in wavelength of a first gain peak, λ peak     —     1 , in the gain profile of said optical signal transported via said first Raman amplification span;  
 adjusting the pump wavelengths of said plurality of Raman pumps by +Δλ period /2, if said first gain peak wavelength, λ peak     —     1 , is longer than a first (shortest) gain peak wavelength, λ 1 , of said gain profile of said optical signal transported via said second Raman amplification span by more than half of said sine function's wavelength period, Δλ period ; and  
 adjusting the pump wavelengths of said plurality of Raman pumps by λ period /2, if said first gain peak wavelength λ peak     —     1  is shorter than said first gain peak wavelength λ 1  by more than half of said sine function's wavelength period, Δλ period .  
 
   
   
     15. The method of  claim 1 , wherein said gain profile of the optical signal transported via said second Raman amplification span is a stored, predetermined expected gain profile. 
   
   
     16. The method of  claim 1 , wherein said gain profile of the optical signal transported via said second Raman amplification span is a gain profile determined by a Raman pump controller. 
   
   
     17. An apparatus comprising a memory for storing program instructions and information, such as gain profiles, and a processor for executing said instructions, said apparatus adapted to perform the steps of:
 determining a gain profile of an optical signal propagated via a first Raman amplification span in a Raman-amplified WDM system;  
 comparing said determined gain profile to a gain profile of a second Raman amplification span;  
 determining, from said comparison, an amount of wavelength adjustment required for a plurality of Raman pumps comprising at least one Raman pump block of said second Raman amplification span to alter the gain profile of said second Raman amplification span, such that a cumulative gain profile of an optical signal transported via said first and said second Raman amplification spans approaches a desired gain profile, wherein said desired gain profile is a gain profile tending to reduce the gain ripple of at least said optical signal in said first Raman amplification span; and  
 generating a control signal suitable for adjusting the wavelengths of said plurality of Raman pumps of said at least one Raman pump block an amount consistent with said determined amount.  
 
   
   
     18. The apparatus of  claim 17 , further configured to adjust, substantially simultaneously, the wavelengths of said plurality of Raman pumps of said at least one Raman pump block an amount consistent with said determined amount. 
   
   
     19. The apparatus of  claim 17 , further configured to determining a cumulative gain profile of an optical signal transported via said first and said second Raman amplification spans. 
   
   
     20. The apparatus of  claim 17 , wherein said gain profile of the optical signal transported via said second Raman amplification span is a predetermined expected gain profile stored in said memory. 
   
   
     21. The apparatus of  claim 17  wherein said gain profile of the optical signal transported via said second Raman amplification span is determined by said apparatus. 
   
   
     22. A Raman-amplified WDM system, comprising:
 at least two Raman amplification spans, each span comprising, 
 at least one Raman amplification fiber, and  
 at least two Raman pumps, comprising at least one Raman pump block, for pumping said at least one Raman amplification fiber,  
 
 a first detector, for measuring the spectral properties of an amplified optical signal in a first Raman amplification span of said at least two Raman amplification spans; and  
 a Raman pump controller, adapted for: 
 determining a gain profile of an optical signal propagated via said first Raman amplification span using the spectral properties measured by said detector;  
 comparing said determined gain profile to a gain profile of a second Raman amplification span of said at least two Raman amplification spans;  
 determining, from said comparison, an amount of wavelength adjustment required for said at least two Raman pumps comprising said at least one Raman pump block of said second Raman amplification span to alter the gain profile of said second Raman amplification span, such that a cumulative gain profile of an optical signal transported via said first and said second Raman amplification spans approaches a desired gain profile, wherein said desired gain profile is a gain profile tending to reduce the gain ripple of at least said optical signal in said first Raman amplification span; and  
 generating a control signal suitable for adjusting the wavelengths of said at least two Raman pumps of said at least one Raman pump block an amount consistent with said determined amount.  
 
 
   
   
     23. The Raman-amplified WDM system of  claim 22 , wherein said Raman pump controller is further adapted to adjust, substantially simultaneously, the wavelengths of said at least two Raman pumps of said at least one Raman pump block an amount consistent with said determined amount. 
   
   
     24. The Raman-amplified WDM system of  claim 22 , wherein said first Raman amplification span and said second Raman amplification spans are consecutive, Raman amplification spans. 
   
   
     25. The Raman-amplified WDM system of  claim 22 , further comprising a second detector for determining a cumulative signal gain profile of an optical signal transported via said first and said second Raman amplification spans. 
   
   
     26. The Raman-amplified WDM system of  claim 22 , wherein said first Raman amplification span and said second Raman amplification spans are non-consecutive, Raman amplification spans. 
   
   
     27. The Raman-amplified WDM system of  claim 26 , further comprising a second detector for determining a cumulative signal gain profile of an optical signal transported via said first and said second Raman amplification spans and any intermediate Raman amplification spans. 
   
   
     28. The Raman-ampified WDM system of  claim 23 , wherein said Raman pump controller is further adapted to determine a cumulative signal gain profile of an optical signal transported via said first and said second Raman amplification spans and any intermediate Raman amplification spans. 
   
   
     29. The method of  claim 1 , wherein said desired gain profile is a gain profile tending to reduce a total gain ripple of said Raman-amplified WDM system.